The present invention relates to a protective device for safeguarding a hazardous area, such as the hazardous area of an automatically operating machine. The invention particularly relates to a device having an image recording unit for recording an object image, which unit includes an image sensor having a plurality of light-sensitive pixels, an evaluation and control unit which triggers a safety function responsive to the recorded object image, and a testing device for checking the functional reliability of at least the image recording unit.
The invention moreover relates to a method of checking the functional reliability or proper function of an image sensor which includes a large number of light-sensitive pixels, the image sensor recording an object image during operation. A defined test image is supplied to the image sensor for checking the functional reliability, and the test image recorded by the image sensor is compared with a defined expectation.
EP 0 902 402 A2 discloses a protective device designated as monitoring system, which device comprises at least one digital camera having a camera module, an A/D converter, and also an evaluation unit with a microprocessor and an image memory. In the evaluation unit, gray values from a currently recorded object image are compared pixel by pixel with gray values of a reference image. On the basis of this comparison, the protective device is capable to detect whether there is a dangerous foreign object in the monitored area. In response thereto, the closing of the doors of an elevator is prevented then.
Since failure of the protective device due to a fault can result in a situation which is dangerous to humans or material values, the known protective device monitors the functional reliability of the camera. Various methods are proposed for this purpose, namely checking for the presence of a synchronization signal from the camera, checking absolute and/or relative values of the lightness, periodically feeding in a test image to a video multiplexer input of the camera and specifically blinding the camera by means of a light-emitting diode, the image pattern recorded by the camera in this case being compared with a reference pattern.
By means of these methods, it is basically possible to carry out simple checking of the protective device for functional reliability. However, the aforementioned methods are not sufficient, in the form disclosed, to satisfy the requirements of higher safety categories of European Standard EN 954-1, in particular the requirements of categories 3 and 4. For example, by monitoring a synchronization signal in the camera, it is not possible to determine whether individual pixels of the image sensor used in the camera are faulty. Feeding in test images and blinding the camera by means of a light-emitting diode possess, by contrast, the disadvantage that the protective device is “blind” during the time of the functional check. Safeguarding a hazardous area without time gaps is therefore not possible.